1. Field
This invention generally relates to fiber-reinforced molding compositions which provide improved surface characteristics to the molding composites formed therefrom. More specifically, the use of lactone monomers in a fiber-reinforced molding composition results in molding composites having an enhanced surface appearance.
2. Prior Art
Unsaturated polyester resins have been used in a variety of fiber reinforced materials. These materials have been fabricated by differing techniques including hand lay-up, spray-up, filament winding, continuous pultrusion, continuous laminating and matched metal-die molding. Recently, the industry has experienced tremendous growth in the use of unsaturated polyester resins in matched metal-die molding applications. This is particularly true for fiber-reinforced, unsaturated polyester resins employed in the automotive industry.
Despite showing considerable utility in matched metal-die molding applications, standard polyester resins exhibited a number of problems. Major among these problems were warpage of molded parts, poor surface appearance, internal cracks and voids and the inability to reproduce the surface contours of the mold.
Over the years a variety of techniques have been employed to overcome these problems. Examples include the use of fillers, various changes in the resin structure and comonomer choice, and partial polymerization of the resin prior to molding the composite, called B-staging.
The solution which has received the greatest acceptance is the use of certain thermoplastics during the formation of the molding composite to compensate for the polymerization shrinkage associated with the curing of the unsaturated polyester resin. During the cure of the unsaturated polyester resin these thermoplastics, referred to as low profile additives, become incompatible with the thermosetting matrix and, under the heat and pressure of the molding operation undergo a thermal expansion which compensates for the polymerization shrinkage.
The development of low profile additives for fiber-reinforced, unsaturated polyester resin molding applications has expanded the acceptance of these molding materials due to their good surface appearance, dimensional stability, physical properties, assembly consolidation and potential weight savings. However, while fiber-reinforced, unsaturated polyester resin molding has penetrated the industry into many applications, the quality demands for a composite so produced has steadily been increasing. This is especially true with respect to the surface characteristics of the composite.
Despite the attempts mentioned above, there continues a need for formulated molding compositions which will provide improved surface characteristics in the molded composite.